This invention relates to filters and, in particular, to systems and methods for use in implementing an improved multi-coupler in wireless communication systems and, in particular, improvements in capacitive air gap cavity filters utilized to implement a multicoupler.
Referring to FIG. 1, conventional systems include an air variable capacitor filter 100 which includes an LC inductively coupled circuit 101 for each of the air variable capacitor gap cavities. In a typical arrangement there may be any number of air variable capacitor cavities capacitively (not shown) and/or inductively 102 coupled together in series and/or in parallel arrangement. In one conventional multicoupler 103, there may be as many as four air gap filter units 100A, 100B, 100C, 100D coupled together in a rack mount unit 110. The multicoupler 103 couples each of the air gap filter units to provide filter characteristics having bandpass filters at spaced frequencies. For example, where four air variable capacitor cavities are utilized, the multicoupler 103 may couple four air variable capacitor cavities to form four spaced bandpass regions. As shown in FIG. 3, the regions may be between 225 megahertz and 400 megahertz such as, for example, at 232, 250, 275, and 300 megahertz center frequencies.
A conventional multicoupler may include a mechanical connection at the antenna input port to short the antenna input when a capacitive air gap cavity filter is unplugged from the multicoupler to allow other capacitive air gap cavity filters to continue to operate.
FIG. 3 shows the four air gap capacitive filters having a bandpass B1, B2, B3 and B4. With an air variable capacitor filter, each filter may be tuned to a different center frequency by adjusting, for example, the capacitance. The dotted line in FIG. 3 shows the filter B1 being tuned through the center frequency range of the B2 filter. However, a problem will occur where the B1 filter passes through the B2 filter resulting in an impedance mismatch. In the event of an impedance mismatch, when the B1 filter is tuned through the center frequency range of the B2 filter, a high insertion loss and a high reflection occurs. This results in a loss of the signal for an instant in time until the B1 filter is tuned beyond the B2 filter. Accordingly, a problem exists with the prior art which has not been heretofore solved by using conventional tuning techniques.
Where a plurality of filters are coupled to a single antenna with a single signal splitter in the rack mount which splits the antenna signal into four separate bandpass filters with separate outputs and a single input from the antenna, the high reflective and/or high insertion loss as the two filters are tuned past each other may cause the user application software to totally shut down as a result of the error condition by having a signal going into the filter and no signal coming out of the filter. This may cause severe problems in some systems including loss of data and/or loss of operational capability for the system.
Accordingly, a solution is required to overcome the above mentioned problems.